1. Field of the Invention
The present invention relates to a power converter including a complementary regeneration circuit for eliminating oscillations and conserving leakage energy to increase efficiency and save energy.
2. Description of the Related Art
Computers and computer systems are becoming smaller and more sophisticated and yet are operating at higher frequencies. Notebook computers, for example, typically include an i486 microprocessor by the Intel Corp. (Intel), which may operate at frequencies of up to 100 MHz. The size of the notebook computer, however, has about the same dimensions as a stack of notebook paper being between 1 to 2 inches thick. Power may be provided from an AC source, such as a typical wall outlet, or by batteries. The present invention primarily concerns the AC adapter, where AC voltage and current from an AC source is converted to regulated DC power appropriate for use by the computer system.
In general, it is desired to convert the same amount of power to operate the computer as a conventional AC/DC converter, except using a smaller and lighter power supply while also decreasing heat generation. In practical terms, the same level of power conversion is desired at the same ambient temperature without the use of fans within the power supply and without negatively impacting the internal or external thermal environment. Therefore, the power supply must be physically smaller and yet achieve a higher efficiency, thereby requiring a higher power density. A higher efficiency is necessary to maintain the temperature requirements without an increase in size.
It is further desired to eliminate the bulky AC adapter typically provided with a portable or notebook computer. The AC adapter is usually a separate unit, causing inconvenience in use and travel. Thus, it is desired to place the AC adapter inside the housing of the computer itself. Although ambient temperature requirements to prevent harm to human operators are essentially eliminated, the temperature must still be controlled to prevent an undesirable rise in temperature within the housing, which could affect the operation of the computer. Also, the AC adapter should be as lightweight as possible to reduce the overall weight of the computer system. Further, it is desirable that the system operate without excessive noise or electromagnetic interference (EMI) problems.
In summary, it is desired to have the best of all worlds, that is, a low cost, high efficiency, smaller power supply for converting the same or even increased amount of power yet operating without substantial EMI.
To achieve some of these goals, designers have typically favored flyback converters. Flyback converters are simpler and easier to design than other types of converters, including forward converters. The simplicity of the flyback topology allows fewer parts and lower cost, which is ideal for use in smaller computer systems. Other converter methods typically require extra inductances, thereby increasing the size and cost of the AC adapter. For example, forward converters are more efficient on the average, but are generally more complicated. Thus, flyback converters are preferred for use in the smaller, higher powered computer systems.
Flyback converters transfer energy from a primary to a secondary circuit rather than transforming voltage and current. This intrinsic characteristic of flyback converters allows a natural constant power operation, which provides the advantage of reducing size and decreasing the maximum power rating to achieve the same amount of power conversion. The advantages of constant power operation are described in U.S. patent application Ser. No. 07/701,657, filed May 16, 1991, which is hereby incorporated by reference.
In spite of the advantages of flyback converters, they also have several disadvantages. Flyback converters tend to be less efficient than other types of converters and emit proportionately higher amounts of electromagnetic energy. Flyback converters have an average efficiency rating of approximately 70%, although well-designed units may achieve efficiencies of 80-82%. These efficiencies do not compare well to the 85% or above range typically achieved using forward converters. Other known design techniques may be used to increase the efficiency of a flyback converter to slightly above 85%, but these techniques almost invariably add cost and increase size.
The reason for the low efficiency rating of flyback converters is the method of power conversion combined with parasitic reactances. The primary switch is usually a metal-oxide-semiconductor field effect transistor (MOSFET), which includes an undesirable, yet inherent drain to source capacitance. In a flyback converter, energy conversion occurs in two phases for each power cycle, as controlled by a standard pulse-width modulation (PWM) circuit. A bridge and filter circuit converts AC voltage from an AC source to a relatively high, unregulated DC voltage. During a first primary conduction phase, the primary switch is activated allowing a linearly ramping current to flow from the primary DC source through the primary inductance of the transformer. The secondary circuit includes a rectifier diode connected so as not to allow current flow in the secondary inductor during the primary conduction phase. Thus, energy is stored in the transformer during the primary conduction phase.
The PWM circuit monitors certain conditions either in the primary or secondary circuit to determine when to end the primary conduction phase. When these conditions are met, the PWM circuit turns off the primary switch initiating a flyback phase, where the energy stored in the transformer is transferred to the secondary circuit and eventually to the load. Usually, sometime after the end of each flyback phase, a new power cycle is initiated by turning on the primary switch. At this point, the high voltage input source is still present across the primary switch. Thus, the primary switch is activated in a high voltage condition, causing power and efficiency loss as the energy stored in its output capacitance is discharged.
In an ideal transformer, all of the energy from the flow of the primary current is stored in the transformer, and then all the stored energy is transferred to the secondary so that the primary current flow falls to zero instantaneously. However, physical transformers include parasitic, uncoupled ("leakage") inductance in the primary inductor. During the transition switching time, the current through the primary and leakage inductors continues to flow, thereby charging the switch capacitor. Then, the excess energy oscillates naturally between the leakage inductor and the switch capacitor causing unwanted damped oscillations in the primary circuit. When the secondary current falls to zero during the flyback phase, the primary switch capacitor discharges through the primary and leakage inductors causing more unwanted oscillations. Further, the forced switching and coupled inductances cause current spikes in the primary and secondary circuits. These oscillations and current spikes cause most of the noise, regulation and efficiency problems experienced with flyback converters.
The damped oscillations cause several problems. First, during the initiation of the flyback phase, the leakage inductance causes the voltage overshoot across the primary switching device producing excess stress, energy loss and radio frequency noise. The excessive voltage overshoot requires the use of a switching device with twice the voltage rating than theoretically necessary and also the use of EMI filtering devices many times larger than otherwise possible. The MOSFET switching device typically includes a drain to source resistance, referred to as RDS.sub.ON, which is proportional to its voltage rating. Thus, the higher voltage rating leads to a corresponding energy loss in the RDS.sub.ON resistance, requiring larger heat sinks.
The oscillations are also absorbed through stray resistance, causing undesirable heat and leading to a significant loss of efficiency. Any noise or oscillations generated in the primary circuit causes electromagnetic interference (EMI), which could couple into the AC source. This could violate federal radiation emission requirements, or otherwise force a lower EMI rating, and also cause interference with other electrical devices located nearby.
The oscillations generated in the primary circuit are transferred to the output circuit, typically through auxiliary inductances used as a power source for support and control circuitry. This may not pose a real problem during full load conditions since the oscillations are mostly absorbed by the load. However, the excess energy of the damped oscillations transferred to the output tends to cause severe regulation problems during low or no-load conditions. Under low or no-load conditions, excess energy is stored in the load capacitor, causing its voltage to rise. Although only a very small amount of energy is stored during each cycle, the cumulative energy in high frequency operations, typically ranging between 60 to 200 KHz, is substantial over time. The voltage across the load capacitor eventually rises above the allowed maximum voltage level, possibly causing a hazardous condition and damage to circuit components.
Several solutions have traditionally been tried to solve the inherent problems with flyback converters. Although each of these problems may be solved in one way or another, the simultaneous solution to all problems has only been achieved at a significant cost or increase in size of the converter. To solve the noise and EMI problems, filter circuits have been used, which add cost and increase the size due to filter inductances. Voltage overshoot problems may also be solved through filter circuits, damping circuits or snubber circuits, but these circuits typically absorb energy, causing heat. The capacitance of traditional snubber capacitors must be limited since increasing the snubber capacitance reduces the efficiency. The excess energy at the output may be solved through a bleed resistor or other small load, but this again causes heat and a loss of efficiency. Of course, other buck-based topologies or forward topologies have been tried, resulting in a loss of the intrinsic advantages of the flyback topology, such as the constant power benefits.
Therefore, it is desirable to retain the advantages of a flyback converter, such as low cost and simplicity, while reducing or otherwise eliminating the undesirable characteristics, such as oscillations and low efficiency. It is desired to reduce or eliminate these problems without substantially increasing cost or component count to achieve a lightweight, highly efficient AC adapter, that will, ideally, be even smaller than present convention.